[PATCH v1 1/3] Feature test macro overhaul

Corinna Vinschen vinschen@redhat.com
Mon Feb 15 13:41:00 GMT 2016

On Feb 12 12:07, Yaakov Selkowitz wrote:
> On 2016-02-12 08:58, Corinna Vinschen wrote:
> >On Feb 12 07:59, Sebastian Huber wrote:
> >>its good that you keep the FreeBSD __*_VISIBLE defines in place, so
> >>that its still easy to use code from FreeBSD. At least for RTEMS this
> >>is important, since we use the network, USB and SD/MMC stack from
> >>FreeBSD.
> >>
> >>I am not sure if its a good thing to do the feature checks differently
> >>in each C library. My approach would have been to us an existing
> >>mechanism and in case of errors report them to the upstream.
> >
> >I'm also a bit concerned here.  It decouples us partially from upstream.
> >The changes don't look bad, mind, but it now looks a bit like a mix
> >between Glibc and FreeBSD.  Can we maintain this easily in future?
> Having built thousands of packages for Cygwin, I can attest that our feature
> test macros are lacking in several ways:
> * combining -ansi/-std=c* -D_*_SOURCE doesn't always work as expected;
> * several _*_SOURCE switches aren't recognized;
> * some symbols don't have the proper guards.
> Obviously, this is modeled after glibc's feature test macros documentation:
> http://man7.org/linux/man-pages/man7/feature_test_macros.7.html
> with some changes found upon testing, while maintaining the __*_VISIBLE
> nomenclature already present instead of the __USE_* mentioned therein.
> WRT style, I thought that being able to use #if instead of #ifdef (or even
> #if defined() || defined() etc.) would be more readable, particularly when a
> symbol can be switched on my multiple guards. Similarly, e.g.
> __POSIX2001_VISIBLE would be easier to read than __POSIX_VISIBLE >= 200112L,
> particularly in the cases where things were in previous versions of POSIX.1
> but removed in later versions.

IMHO, for compatibility reasons the former __POSIX_VISIBLE and
__XSI_VISIBLE setting should stay available, e.g.,

  #if (_POSIX_C_SOURCE - 0) >= 200112L
  #define __POSIX2001_VISIBLE    1
  #undef __POSIX_VISIBLE
  #define __POSIX_VISIBLE 200112L
  #define __POSIX2001_VISIBLE    0

If you reorder the settings slightly you should be able to do that
without having to #undef

> WRT compatibility with FreeBSD, the only issue would be if entire headers
> are copied therefrom.  However, there are certainly some FreeBSD-isms have
> to anyways be tweaked when doing so; I don't see how this is any different.

Ok, go ahead.  If there's no other complaint about this change in
general, it should be ok to improve the feature test macros as per
your proposal.


Corinna Vinschen
Cygwin Maintainer
Red Hat
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